OS X is the newest of Apple Inc.‘s Mac OS line of operating systems. Although, under its original name of Mac OS X, it was officially designated as simply “version 10” of the Mac OS, “version 9” had a completely different codebase as well as dramatic changes in user interface. Mac OS had been Apple’s primary operating system since 1984, and the family was backward compatible, so OS X could virtualize Mac OS 9 until version 10.5.
Unlike its predecessor, OS X is a Unix operating system built on technology that had been developed at NeXT through the second half of the 1980s and up until Apple purchased the company in early 1997. It was first released in 1999 asMac OS X Server 1.0, with a desktop-oriented version (Mac OS X v10.0) following in March 2001. Since then, six more distinct “client” and “server” editions of Mac OS X were released, thereafter starting with Mac OS X v10.7 Lion,OS X Server is no longer offered as a separate operating system product; instead, the server management tools are available for purchase separately. The most recent version OS X 10.9 Mavericks was first made available on October 22, 2013.
Mac OS X v10.5 Leopard, when running on Intel processors, Mac OS X v10.6 Snow Leopard, and OS X v10.8 Mountain Lion are all certified Unix systems, conforming to the Single UNIX Specification.
Releases of OS X prior to 10.9 were named after big cats; the previous version of OS X, 10.8, is named Mountain Lion. At the 2013 Apple Worldwide Developers Conference, Apple indicated that OS X 10.9 Mavericks and future OS X releases would be named after California locations.